Deployment of aided phytostabilisation at field scale : set up and monitoring lessons
Résumé
In the project GREENLAND (FP7, KBBE-2010-4, 266124) project, different phytomanagement options at large field scale were tested to gain information on practical deployment and long term efficiency. Among the network of 13 large trace element (TE) contaminated sites, one located in the Nord-Pas-de-Calais region (France), was implemented in September 2011 with aided phytostabilisation, i.e. the combination of plants and soil amendment to reduce the risks associated to the presence of soil TE. The site (1 ha) is part of a large sediment disposal site which was affected by intensive industrial activities. The objective was to combine aided phytostabilisation with bioenergy production based on Salix cultivation to reduce the environmental risk posed by these sediments and allow the economic valorization of the contaminated sediments via the sale of the produced biomass. Another objective was to test aided phytostabilisation as a strategy to avoid the propagation of the Japanese knotweed, an invasive species that occurred at the sediment landfill site. In a first step, Deschampsia cespitosa, a grass, and Thomas basic slag (TBS), a basic mineral amendment, already tested at field site on an experimental TE contaminated landfill site, were used to stabilise the TE of the top sediment. In a second step, a SRC composed of two Salix cultivars ('Inger' and Tordis') was deployed to produce biomass. First results show the success of the plant cover that rapidly reached 100% and the success of the Salix plantation although phytoxicity signs appeared after few months. Several hypotheses were studied of which the grass competition for water and essential nutrients. As the sediment landfill site is highly contaminated with metals, mainly Zn and Cd, TBS is expected to decrease the metal shoot transfer towards the grass and the two planted Salix clones and to decrease the metal labile pool. At this stage of the project monitoring the effects of TBS on soil metal mobility or the Salix leaf metal concentrations have not yet been measured. The grass showed very little Zn and Cd concentrations in aerial parts that confirm the choice of this plant as a candidate for phytostabilisation. Conversely, the two Salix cultivars showed very high Cd and Zn leaf concentrations that might not be compatible with a phytostabilisation strategy. The Cd and Zn concentrations in leaves and wood at harvest might direct the conversion process of the produced biomass (thermal treatment, metal recovery). A decrease in Japanese Knotweed was visible after one year of monitoring and corresponded to a reduction in coverage of 27% of the surface area showing that the Japanese Knotweed is less competitive and its growth decrease represents a beneficial effect of phytostabilization in terms of ecological services. The whole chain of the aided phytostabilisation, i.e. from field preparation to the conversion process of the produced biomass, is addressed by this project. Results will be presented step by step taking into account practical experiences and scientific knowledge as well as regulation and economical aspects.